Research from several laboratories have shown that the auditory system can be made more resistant to the effects of noise by prior exposure to lower levels or "conditioning" noise exposures. This proposal has the general goal of elucidating the acoustic parameters of the "conditioning" exposure that leads to a more resistant auditory system. There are six specific objectives (1) What parameters optimize the "conditioning" effect?; (2) What is the bandwidth of the increased resistance?; (3) How permanent is the state of increased resistance?; (4) How long does the "conditioning" exposure need to be and what is the latency of the increased resistance?; (5) Does the increased resistance protect the auditory system from impulse noise? (6) Does the presence of stress related proteins in the cochlea covary with noise exposure. Are these proteins specifically Heat Shock Protein 70 involved in the "toughening process"? The experimental animal model is the chinchilla. Its hearing is measured before and at various times after exposure to the conditioning exposure and the traumatizing exposure. Thirty days after the exposures the animal is sacrificed and the population of sensory, supporting and nerve cells of the cochlea are analyzed. The results may be important for understanding differences in the individual's susceptibility to noise.